rustdoc: Add the ability to input json

This modifies the command-line usage of rustdoc to intake its own JSON output as
well as a rust source file. This also alters the command line from
`rustdoc input file`  to `rustdoc file` with the input/output formats specified
as -r and -w, respectively.

When using a JSON input, no passes or plugins are re-run over the json, instead
the output is generated directly from the JSON that was provided. Passes and
plugins are still run on rust source input, however.
rustdoc: Update the man page

Closes #9622